1. Field of the Invention
The invention relates to rotary drum dryers having shielding flights which are located in the combustion zone of the drum and which in use shield the drum shell from radiant heat from the burner flame supplying heat to the drum and, more particularly, relates to a method and apparatus for cooling such shielding flights using materials in the drum.
2. Discussion of the Related Art
Many asphalt production plants include a rotary drum dryer in which virgin aggregate is heated and dried and then mixed with liquid asphalt. Such dryers typically comprise a rotating drum which is inclined with respect to the horizontal and which has a virgin aggregate inlet in the upper end thereof and a virgin aggregate outlet in the lower end thereof. A burner is mounted adjacent one of the ends so as to direct a flame generally axially into the drum for heating and drying the aggregate flowing therethrough. The burner may be mounted either on the lower end of the drum, thereby producing a counterflow dryer, or on the upper end of the drum, thereby producing a parallel flow dryer. In addition, a fixed sleeve may be mounted around the outlet end of the drum to define a mixing chamber in which the heated and dried aggregate may be mixed with recycled asphalt product (RAP), liquid asphalt, or the like. The combination of such a rotary drum and a fixed sleeve is commonly known as a dryer drum coater or a drum mixer.
Rotary drum dryers of the type described above, whether used in asphalt production plants or in soil remediation or other plants, are functionally separated into a combustion zone located in the vicinity of the burner flame and a drying zone extending from the combustion zone to the remote end of the drum. Shielding is required around the inner periphery of the combustion zone to prevent the intense heat radiating from the burner flame from damaging the shell of the drum. This shielding was traditionally performed by a refractory liner. More recently, however, this shielding has been performed by shielding flights mounted around the inner periphery of the combustion zone of the drum such that the flights shield the drum shell from radiant heat from the burner flame, thereby obviating the need for a refractory liner. The flights are typically tee shaped and include a shielding member extending generally parallel to the adjacent portion of the drum shell and a post extending radially from the shielding member to the drum shell. Examples of rotary drum dryers having such flights are disclosed in U.S. Pat. Nos. 4,189,300 to Butler (the Butler patent) and 5,203,693 to Swanson (the Swanson patent).
Since the shielding flights are exposed to the radiant heat of the burner flame in the combustion zone, the flights become overheated and rapidly deteriorate and must be frequently maintained or replaced, thus requiring significant undesired downtime. Attempts have been made to alleviate this problem by providing devices to cool the flights using the aggregate in the drum.
For instance, the system proposed in the Butler patent employs shielding flights having radially outwardly projecting legs defining pockets between the radial outer surface of the flights and the shell of the drum. The pockets scoop up aggregate as the flights traverse the lower portion of the drum and hold the aggregate on the flights through much of the drum rotation such that the retained aggregate receives heat from the flights to cool the flights. This cooling is, however, limited because aggregate is for the most part held on the flights rather than cascading over the flights. Essentially the same portions of aggregate thus receive heat from the flights through substantially the entire cooling cycle and thus themselves tend to become overheated. Moreover, although the radially projecting legs defining the pockets are designed to inhibit the showering of materials into the burner flame, a significant amount of such showering may nevertheless occur, thus at least partially quenching the burner flame and decreasing burner efficiency and resulting in undesired emissions.
The process disclosed in the Swanson patent employs specially shaped shielding flights each having a radially outwardly angled leading edge and a radially inwardly angled trailing edge. The inwardly angled leading edges dig into the aggregate and cause the flights to be covered by aggregate as they rotate through the bottom portion of the drum. The inwardly angled trailing edges retain aggregate for a limited time as the flights rotate beyond the bottom portion of the drum; they then direct the retained aggregate back onto the aggregate accumulated in the lower portion of the drum before it can be lifted into the burner flame, thus cooling the flights without significantly quenching the burner flame. The cooling provided by this process is, however, necessarily limited by the limited angle of rotation through which it occurs.